Tanning process



United States Patent Int. Cl. C14c 3/24 US. Cl. 894.26 14 Claims This invention pertains to a tanning process and more particularly to a tanning process using a lignosulfonate salt as a tanning agent.

There are two principal tanning processes being presently used. One is a vegetable tanning process and the other is a metallic salt tannage with chromic salts being employed in most cases. The processes produce characteristic leathers of which each has some property superior over the other. In an attempt to manufacture leather with the best properties of each, many tanners frequently resort to the re-tan system which often embraces a preliminary chrome tannage followed by a partial or complete vegetable tannage. By this dual process, the tanner can take advantage of the rapidity of the chrome tannage, its smooth grained eifects and its high heat resistance, and also obtain the mellow, light colored, full, round leather characteristics of vegetable tannage. Attempts to employ chromium salts of syntans and vegetable tanning agents to obtain the combined effects of chromium and vegetable tannage in a one-step tanning process heretofore have not been too successful. The leather obtained usually has the characteristics of chromium tannage, being thin and lacking the desired color and fullness of leather obtained by vegetable tanning. Also, in some cases, shrinkage of the surface of the leather may be obtained which is detrimental to the leather and depreciates its value.

It is, therefore, an object of this invention to provide a tanning process to obtain a leather having the high shrinkage temperature characteristic of chrome leather with the light color and fullness characteristic of vegetable tanning. A further object is to provide a tanning agent which imparts the desirable features of chrome tanning and of vegetable tanning to the leather. A still further object is to provide a process wherein a leather can be obtained having the desirable properties of chrome tanning and vegetable tanning in a one-step tanning process.

The above and other objects are attained according to this invention by contacting the hide with an aluminumchromium lignosulfonate salt which contains a relatively minor amount of trivalent chromium. The employment of the aluminum salt containing the minor amount of chromium as a tanning agent produces a leather which has a high shrink temperature characteristic of chrome leather and has the light color, the fullness and the plumpness of vegetable tanned leather. The hides can be tanned in one step without the necessity of retanning to obtain leather having the desired characteristics.

The tanning agent must contain at least 50% of a stoichiometric amount of aluminum and a relatively small amount of trivalent chromium of about to 50% of the stoichiometric amount, based upon the lignosulfonate radicals. A salt containing from 75 to 100% of the stoichiometric amount of aluminum and chromium in the range of to 25% of the stoichiometric amount is preferred. An excess of aluminum up to about twice the normal stoichiometric amount may be used without obtaining an appreciable adverse effect. It is believed that the chromium, when added in small amounts, somehow complexes with the aluminum and lignosulfonate to obtain a product giving the desired result. Generally, at the low chromium additions, an increase in the chromium content tends to enhance the desirable chromium tanning characteristic in the leather. However, the enhancement levels off at about 25% of the stoichiometric amount with the undesirable features of chrome tanning becoming more apparent when the amount of chrome is increased above 50% of the stoichiometric amount.

The term stoichiometric amount, as used herein, means the chemically equivalent amount of metal necessary to react with the sulfonate radicals to form simple salts. This amount can be determined by various analytical methods. However, for pulping liquors, a simple base exchange reaction where one of the products formed is relatively insoluble may be used. Where, generally, pulping liquor may contain a small amount of other organic acids in addition to the lignosulfonic acid which may undergo the base exchange reaction, the amount of these organic acids is not sufficient to materially effect the results. For example, for a calcium base spent sulfite liquor, the stoichiometric amount can easily be determined by adding sulfuric acid or a soluble sulfate salt under acid conditions, aluminum sulfate for example and determining the amount of calcium sulfate which precipitates out. Likewise, other anions forming insoluble calcium salts as oxalate and the like can be used. For a pulping liquor of a base which may not be conveniently reacted to form an insoluble product upon base exchange, the pulping liquor may be converted to the acid form by ion exchange resin and the amount of metal removed calculated or the acid product can then be converted to the metal salt which will form an insoluble product upon base exchange.

The aluminum-chrome lignosulfonate salt is obtained by the reaction of aluminum and chromium salts with the lignosulfonate. Lignosulfonates are obtained by sulfonation by the various known methods of lignin obtained from any source. Lignin is a polymeric substance of substituted aromatics found in plant and vegetable tissue associated withcellulose and other plant constituents. Thus, vegetable and plant tissue are lignin-containing materials which are the principal sources of lignin.

One of the main sources for lignosulfonate is the residual pulping liquors from the paper and pulp industry where lignocellulosic materials such as wood, straw, corn stalks, bagasse and the like are processed to separate the cellulose or pulp from the lignin. In the sulfite pulping process, the lignocellulosic material is digested with a bisulfite or sulfite to obtain a sulfonated residual pulping liquor commonly referred to as spent sulfite liquor wherein the sulfonated lignin is dissolved. In other pulping processes, the residual pulping liquor as obtained from the process may not be a sulfonated product. However, the residual liquors or products containing the lignin portion of the lignocellulose materials from the other process and also from the sulfite process may be treated by various known methods to sulfonate the lignin to the different degrees desired. For example, the residual liquor obtained in an alkaline pulping process such as kraft, soda or other alkaline processes may be sulfonated by reacting the product with a bisulfite or sulfite to obtain a sulfonated residual pulping liquor. Likewise, lignins known as hydrolysis lignin obtained from the hydrolysis of lignocellulose and hydrotropic lignins derived from the hydrotropic pulping processes may be sulfonated and used.

The lignosulfonate products obtained from the sulfonated pulping process or by the sulfonation of other residual pulping liquors or lignin-containing material may contain many other constituents besides the sulfonated lignin. While these other materials may be removed, it is not necessary to do so and the product can be used without purification and recovery of the liqnosulfonate. Usually in sulfonated residual pulping liquors, at least 40 to 50% of the dissolved solids are lignosulfonates and this proportion can be greatly increased by purification and removal of the non-tanning constituents such as sugars and other carbohydrate materials which may be found, for example, in spent sulfite liquor. Also, the lignosulfonates in the pulping liquors may be fractionated to obtain lignosulfonates of a particular molecular weight. However, generally, the residual pulping liquor is used without further purification other than the removal of the sulfurous acid which may be present, for example, in the spent sulfite liquor.

The sulfonated lignins obtained upon the sulfonation of lignin may be salts of certain cations, such as magnesium, calcium, ammonium, sodium and others. The lignosulfonate salts thus obtained may be used as such to be reacted with aluminum and chromium compounds to obtain the desired salts, or the salts obtained from the pulping process may be converted to lignosulfonic acid by addition of acid or use of ion exchange resins and then reacted to obtain the desired metal salt of aluminum and chromium.

The formation of the aluminum-chrome lignosulfonate salt is generally efiected by merely intermixing a soluble aluminum compound and a soluble trivalent chromium compound with a sulfonated residual pulping liquor. If the pulping liquor used is a calcium base liquor or contains metallic ions which are undesirable in tanning, the reaction with the aluminum and chromium may be carried out under conditions such that the undesirable cations are precipitated out. For example, with a calcium based spent sulfite liquor, aluminum sulfate may be added which will result in the precipitation of the calcium as calcium sulfate. Also, the reaction of the aluminum salt or the chromium salt may be carried out under acid conditions in the presence of sulfuric acid resulting in the precipitation of calcium sulfate. The lignosulfonate may also be treated with an acid to precipitate or an ion exchange resin to remove the undesirable cations prior to reaction.

The nature of the reaction between the aluminum and chromium with the lignosulfonate or sulfonated pulping liquor as well as the nature of the compound formed by their reaction is not definitely known. It is believed that aluminum and chromium ions form complexes with the sulfonated lignin as well as simple salts. The metals may be chemically bound to the lignosulfonate in amounts greater than the stoichiometric amount which would be required for simple salt formation. For example, for a fermented spent sulfite liquor, aluminum in an amount of about 2.3 weight percent or chromium in an amount of about 4.5 weight percent of the spent sulfite liquor solids may be required to replace the calcium found in the spent sulfite liquor. However, larger amounts of aluminum or chromium or both may be added and this extra addition over the calcium equivalence appears to continue to be chemically bound in some manner for the products obtained fail to precipitate out under conditions where aluminum or chromium present as free ions would. Thus, the product appears to have characteristics of a complex salt which is still soluble in an aqueous medium.

Any soluble aluminum compound which supplies aluminum cations may be used in the reaction with lignosulfonate or the pulping liquor. Typical compounds include aluminum salts of the common mineral acids such as sulfuric, hydrochloric, and nitric as well as organic aluminum salts. Any soluble trivalent chromium compound may likewise be used. However, a considerably enhanced product is produced when a trivalent chromium product is obtained by the oxidation of a carbohydrate or carbohydrate-containing material by hexavalent chromium. Sugars such as dextrose, glucose or carbohydratecontaining material as molasses, spent sulfite liquor and the like are illustrative examples of the materials which may be used. The oxidation of these materials in the reduction of the hexavalent chromium forms low molecular weight acids and other products which impart beneficial characteristics to the leather. In the reduction reaction, a carbohydrate material such as spent sulfite liquor is generally used in an amount of from 40 to 250 weight percent of the hexavalent chromium. Amounts less than 40% of the weight of hexavalent chromium may be used; however, a low pH and a long reaction time may be necessary to obtain a substantially complete reduction of the hexavalent chromium to trivalent form. With larger amounts of reducing materials, for example, to 150%, the reaction is relatively rapid and low molecular weight organic acids are obtained which improve the leather. With amounts above 250 weight percent, generally insufficient oxidation of the carbohydrate is obtained to form the desired oxidation products.

No particular process is necessary to effect the reaction of the aluminum and chromium with lignosulfonate. The aluminum and the trivalent chromium compounds may be intermixed together or separately with the lignosulfonate or the sulfonated pulping liquor in the required proportions. For example, when a spent sulfite liquor is used, the aluminum compound may be added to obtain from about 1.7 to 2.3 weight percent aluminum, based upon the spent sulfite liquor solids, and the trivalent chromium compound is added to obtain from .5 to 1 weight percent of trivalent chromium, based upon the spent sulfite liquor solids. The employment of the above amounts generally will result in obtaining the preferred ratios of the particular metals. In carrying out the reaction, the spent sulfite liquor may be evaporated until it contains from 20 to 60% dissolved solids, the soluble aluminum compound added to the solution, and the desired amount of trivalent chromium product then intermixed. It is generally preferred to elfect the reaction at a pH below 6, preferably in the range of 2 to 4, although a lower pH may be used. The reaction may be carried out at ambient temperature or at an elevated temperature for a short period of time of from 5 to 10 minutes. However, the reaction mixture may be heated to a temperature in the range of 70 to C. for about /2 to 2 hours. The product may be used as obtained or further diluted as desired or dried as by spray drying to obtain the product in solid form.

=In tanning with the aluminum-chrome lignosulfonate, the normal processes of vegetable or chrome tannage may be employed. Generally, due to the full fiber development obtained with this tanning agent, a fat liquor designed for vegetable combination leather, containing a high percentage of raw oil, is preferably used. The tanning agent may be used as a dilute solution or as a concentrated solution for low float drum tanning.

EXAMPLE I An aluminum lignosulfonate was prepared by the reaction of a calcium base sulfite liquid with aluminum sulfate. The spent sulfite liquor was fermented to convert the fermentable sugars to alcohol and then stripped to recover the alcohol. To 31.2 kilograms of the fermented spent sulfite liquor, concentrated to 48% solids content, a strong aqueous solution containing 3.75 kilograms of papermakers alum was added. The mixture was stirred for five minutes and then the pH was adjusted to 3.8 by the addition of 3.5 liters of 18% sodium hydroxide solution.

The trivalent chromium to be reacted with the above aluminum lignosulfonate was prepared by reacting sodium dichromate with spent sulfite liquor. Sodium dichromate in an amount of 180 grams was dissolved in one liter of water and the resulting solution was treated with 174 grams of concentrated sulfuric acid. The solution was vigorously stirred while 200 ml. of spent sulfite liquor containing 52% solids were added slowly. The oxidation of the spent sulfite liquor elevated the temperature of the reaction mixture to 84 C. While the reaction was substantially completed in one hour, 50 grams of 50% sodium thiosulfate solution were added to insure complete reduction of the chromium.

The chromium reaction product, after cooling, was intermixed with the neutralized aluminum lign-osulfonate reaction product prepared as described above. The resulting mixture contained about 1% of a trivalent chromium and about 2 weight percent of aluminum, based upon the total solids content. This mixture was then used as a tanning agent for tanning of goatskin.

Pickled goatskin squares in an amount of 200 grams in weight were degreased by drumming in kerosene and subsequently washing them in brine containing a stable, nonionic detergent. The tanning solution was prepared by intermixing 105 grams of the dried aluminum-chrome lignosulfonate product obtained above with 200 grams of weight percent sodium chloride brine. The solution was added to the drum containing the degreased skins and the drum rotated for 18 hours. After the drumming, the tanned leather was removed, washed, fat liquored, set out, tacked and dried. The shrinkage temperature was found to be 87 C. The color was a light pastel butt, and the leather was round and plump.

Additional squares of pickled goatskin were tanned in a manner similar to that described above except that the tanning agent used was prepared by reacting a greater ratio of the trivalent chromium product with the aluminum lignosulfonate. The aluminum chrome sulfate tanning agent when dried contained about 2% of trivalent chromium.

The tanned leather had a shrinkage temperature of 78 C. and was not quite as plump as the leather obtained with the tanning agent containing about 1% of trivalent chromium.

EXAMPLE II A portion of the aluminum-lignosulfonate prepared above containing about 2 weight percent aluminum and 1 weight percent of trivalent chromium was spray dried and used for retanning of chrome tanned split and shaved cowhide. Squares of the chrome tanned leather were cut in matched pairs. One of the pieces of the leather from each of the matched samples was washed and retanned in a 10 weight percent aqueous solution of the aluminumchrome lignosulfonate prepared above. The tanning solution was obtained by adding the spray dried material to Water to obtain the desired concentration. The leather was retanned by drumming of the samples in a 100% float at 85 F. for two hours. Both the treated samples and the corresponding untreated control pieces were then fat liquored, piled, rung, pasted out and dried. They were then washed and air dried to 25% moisture content and finally staked and brought to equilibrium with air at 74 F. and 50% relative humidity.

In the 24 squares tested, an average thickness of 0.085 inch was found for the retanned leather, while the chrome control pieces averaged .074 inch. All of the retanned squares were found to have a finer, tighter break than the control squares. Unlike some retannage, the use of the aluminum-chrome lignosulfonate agent caused no significant decrease in strength as measured :by the stretchtear method.

What is claimed is:

1. A process for the tanning of hides in the preparation of leather, which comprises contacting the hide with a solution of a lignosulfonate salt of aluminum and chromium as a tanning agent, said lignosulfonate salt containing aluminum in an amount of at least 50 percent of the stoichiometric amount and chromium as trivalent chromium in an amount of from 5 to 50 percent of the stoichiometric amount.

2. A process according to claim 1 wherein the lignosulfonate contains from 75 to percent of the stoichiometric amount of aluminum and from 10 to 25 percent of the stoichiometric amount of chromium.

3. A process for tanning hides in the preparation of leather, which comprises contacting the hide with a tanning solution prepared by reacting a sulfonated residual pulping liquor with a soluble aluminum compound in an amount of at least 50 percent of the stoichiometric amount of aluminum, and intermixing a trivalent chromium product in an amount of from 5 to 50 percent of the stoichiometric amount of chromium, said chromium product being obtained by the reduction of hexavalent chromium to trivalent chromium with a carbohydrate.

4. A process according to claim 3 wherein the soluble aluminum compound is added in an amount to obtain from 75 to 100 percent of the stoichiometric amount of aluminum and the chromium product is added in an amount to obtain from 10 to 25 percent of the stoichiometric amount of the chromium.

5. A process according to claim 4 wherein the chromium product is obtained by reaction of a hexavalent chromium compound with a carbohydrate-containing material selected from the group consisting of molasses, and spent sulfite liquor.

6. A process according to claim 5 wherein the sulfonated residual pulping liquor is a spent sulfite liquor.

7. A process for the tanning of hides in the preparation of leather, which comprises contacting the hide with a tanning solution prepared by reacting a spent sulfite liquor with a soluble aluminum compound in an amount of at least 50 percent of the stoichiometric amount of aluminum and intermixing a trivalent chromium product in an amount of from 5 to 50% of the stoichiometric amount of chromium as trivalent chromium, said chromium product being prepared by reducing hexavalent chromium compounds by reaction with spent sulfite liquor in amounts to reduce substantially all of the hexavalent chromium to trivalent chromium and to oxidize the spent sulfite liquor to produce low molecular weight acids.

8. A process according to claim 7 wherein the chromium product is obtained by the reaction of the hexavalent chromium compound with spent sulfite liquor in an amount of spent sulfite liquor solids of from 40 to 250 weight percent of the hexavalent chromium in the chromium compound.

9. A process according to claim 8 wherein the aluminum compound is reacted with the spent sulfite liquor in an amount to obtain from 75 to 100% of the stoichiometric amount of aluminum and the chromium product is intermixed in an amount to obtain from 10 to 25 of the stoichiometric amount of chromium.

10. A process for tanning of hides in the preparation of leather, which comprises contacting the hide with a tanning solution prepared by reacting a spent sulfite liquor with a soluble aluminum compound in an amount to obtain from 1.2 to 2.3 weight percent aluminum, based upon the spent sulfite liquor solids, and intermixing a trivalent chromium compound in an amount to obtain from 0.2 to 2 weight percent of chromium, based upon the spent sulfite liquor solids.

11. A process according to claim 10 wherein the chromium compound is a reduction product of a hexavalent chromium compound with an organic material selected from the group consisting of sugars, molasses, and spent sulfite liquor.

12. A process according to claim 11 wherein the hexavalent chromium compound is reacted with spent sulfite liquor in an amount of spent sulfite liquor solids of from 7, 40 to 250 weight percent of the hexavalent chromium in the chromium compound.

13. A process according to claim 12 wherein the hexavalent chromium compound is an alkali metal dichromate.

14. A process for tanning of hides in the preparation of leather, which comprises contacting the hide with a tanning solution prepared by reacting a calcium base spent sulfite liquor with a soluble aluminum compound in an amount to'obtain from 1.2 to 2.3 weight percent aluminum in the presence of an acidic anion forming substantially insoluble calcium salts to thereby precipitate the calcium, intermixing a trivalent chromium compound to obtain from 0.2 to 2 weight percent of trivalent chromium, based upon the spent sulfite liquor solids, and removing the precipitated calcium compound, said chromium prod- 1 uct being prepared by reacting an alkali metal dichromate with spent sulfite liquor in an amount of spent sulfite liqnor solids of from 80 to 150 Weight percent of the hexavalent chromium in the dichrornate.

References Cited V FOREIGN PATENTS 605,036 11/1934 Germany.

5 DONALD LEVY, Primary Examiner.

U.S. Cl. X.R. 

1. A PROCESS FOR THE TANNING OF HIDES IN THE PREPARATION OF LEATHER, WHICH COMPRISES CONTACTING THE HIDE WITH A SOLUTION OF A LIGNOSULFONATE SALT OF ALUMINUM AND CHROMIUM AS A TANNING AGENT, SAID LIGNOSULFONATE SALT CONTAINING ALUMINUM IN AN AMOUNT OF AT LEAST 50 PERCENT OF THE STOICHIOMETRIC AMOUNT AND CHROMIUM AS TRIVALENT CHROMIUM IN AN AMOUNT OF FROM 5 TO 50 PERCENT OF THE STOICHIOMETRIC AMOUNT. 